This invention relates to color cathode ray tubes of the type having a shadow mask, and especially to a system for suspending a shadow mask on the faceplate of a color tube. This invention has applicability to suspension systems for shadow masks of various types, including post deflection focus masks.
Conventional color cathode ray tubes have a shadow mask assembly which includes a heavy frame to which is welded a dished, apertured mask. The frame is, by design, extremely rigid and provides the necessary rigidity for the mask. The mask-frame assembly is mounted in a conventional tube by a suspension system comprising three or four leaf springs which are welded to the frame at spaced points around the periphery thereof. These springs must be relatively stiff to support the heavy mask-frame assembly, typically applying a load of 4-5 pounds or more to the mask-frame assembly. The springs have apertures at their distal ends which engage studs projecting inwardly from a rearward flange on the tube faceplate when the assembly is mounted in a tube. The mask-frame assembly is capable of being demounted and precisely remounted in a tube by depressing the springs to disengage the said studs. This type of system has proven to be commercially viable, however, the mask-frame assembly and the tube envelope are undesirably expensive.
The present invention involves a radical departure from conventional and other prior art approaches to shadow masks and shadow mask suspension systems. By the present approach, a low cost, lightweight, non-self-rigid, torsionally flexible mask is provided. The faceplate is used to impart the necessary rigidity to the mask. A novel suspension system is provided which furnishes a mechanically rigid link between the faceplate and the mask, and yet which permits the mask to be conveniently and repeatably demounted and precisely remounted in the tube. The advantages of this system are manifold. A primary advantage resides in the appreciable savings in tube cost. Tube cost savings result from the use, in a preferred embodiment, of an envelope having a flangeless faceplate which is less expensive than the conventional flanged faceplate, and from the use of a lightweight (low mass), low cost shadow mask (preferably of one-piece, frameless construction).
The system with which this invention is involved has imposed upon it a number of requirements and constraints not presented in conventional systems in which a rigid frame is used to impart rigidity to the mask. Before enumerating these requirements and constraints, a discussion of certain underlying principles will be engaged. A shadow mask of the type with which this invention is concerned may be modeled as a rectangular four bar linkage affixed to a flexible sheet. Such a model is shown in FIG. A. The four rigid bars of the linkage are designated A, B, C and D; the sheet is labeled S. As is well known, a four bar linkage is not inherently a rigid structure. The rectangular four bar linkage, in its free state, might, e.g., quite easily be skewed into a parallelogram geometry. It is evident, however, that the FIG. A model cannot be skewed in its plane to take a parallelogram shape since it is affixed to the sheet S.
The linkage, can, however, be torsionally twisted about its diagonals, as shown for example in FIG. B. In FIG. B, the model has been twisted as follows -- the linkage bar A has been rotated toward the reader (see arrows); the linkage bar C has been rotated away from the reader. The corners 1 and 3 have been displaced upwardly and the corners 2 and 4 have been displaced downwardly. The sheet S is thus stressed convexly along diagonal 2-4 and somewhat concavely at the ends of diagonal 1-3. The model may thus be thought of as being twisted about one of its diagonals (here shown as diagonal 1-3). It can be noted that the model configuration, after twisting, is changed substantially less along its major axis M.sub.a and minor axis M.sub.i, than along the diagonals. Thus a four bar linkage affixed to a flexible sheet is relatively stiff with respect to its major and minor axes (due to the rigidity of the bars), but is relatively flexible in torsion. When torsionally flexed (twisted), about its diaganols, the corners are displaced, but points on the major and minor axes remain relatively stationary.
As will be pointed out in more detail hereinafter, the shadow mask with which this invention is concerned is similar to the described model in its mechanical characteristics.
The principles of this invention, though not limited to such application, are most useful when embodied in a color cathode ray tube having a flangeless faceplate. When such a faceplate is sealed to mating funnel after completion of the faceplate screening and mask insertion operation, the faceplate is very apt to experience a twist-wise elastic distortion due to a tolerance-related configurational mismatch between the funnel and faceplate sealing surfaces. Any such distortion will be rendered a permanent deformation when sealing cement has cured and the selaing operation is completed. Thus, one of the necessary general requirements imposed on a mask and mask-suspension system intended for use with a flangeless faceplate is that it must be able to adapt to such twist-wise deformations of a faceplate with which it is mated. Stated another way, the mask must be capable of flexing or twisting about its diagonals in much the same way faceplates are apt to twist-wise deform in their contour during tube fabrication, and its suspension system must provide for such adaption. As will become evident as this description proceeds, the shadow mask and suspension system with which this invention is concerned are uniquely capable of meeting this requirement.
Second, and of equal significance -- with respect to any given faceplate, since the mask is non-self-rigid, the suspension system for the mask must effectively transfer the rigidity of the faceplate to the mask.
Third, the suspension system must precisely fix and hold a predetermined spatial position of the mask as a whole relative to the faceplate against translational or rotational displacement, in spite of any thermal expansion or contraction of the mask, demounting and remounting of the mask, or mechanical shocks.
Fourth, it is desirable that any thermally induced movement of any part of the mask or of any mask suspension element during tube operation be radial, rather than tangential, since radial errors can be compensated by adjusting in the beam deflection characteristic, whereas tangetial errors cannot be.
Fifth, it is desirable that the system permit the mask to be conveniently and quickly demounted and remounted, preferably automatically, since in conventional factory faceplate screening practices the mask is mounted on or demounted from the faceplate many times.
A sixth general requirement is that the mask suspension system should carry a low manufacturing cost.
As will be pointed out in more detail hereinafter, this invention involves the provision of a shadow mask suspension system comprising four suspension devices. One at each corner of the tube faceplate, at least three of the devices including an axially extending cantilevered leaf spring. I have found that numerous additional specific requirements are imposed upon such a system.
A seventh specific requirement is as follows. In order to achieve the afore-discussed fixing of the spatial position of the mask, in the context of a four-corner cantilevered spring suspension system, as described, it has been discovered that at least three of the springs must be extremely stiff in the tangential direction and the correction of the spring to its supporting instrumentality must also be extremely rigid in the tangential direction. If such is not the case, the mask will not always return to its bogey position (nominal assigned position) after having received a mechanical shock or after having been demounted and remounted.
The present invention provides a disengageable spring suspension system by which a non-self-rigid shadow mask is supported at its four corners, and particularly centers on structure for mounting a spring or alternatively for engaging a spring, in a system of the type described.
No prior art is known which is capable of meeting the requirements and constraints afore-stated. A U.S. Pat. No. to Hafkenschied et al -- 3,573,527, discloses a conventional type mask-frame assembly suspended by three edge-bonded leaf springs -- two attached in adjacent corners of the assembly and the third on the opposed side thereof. In the interest of establishing a rigid connection, the corner-attached springs are welded to the mask frame at four points -- two on adjoining side surfaces of the frame and two on a back surface of the frame. A comparison of the system of this invention with the Hafkenschied et al system, however, will show them to be very different in concept and execution and will reveal the Hofkenschied et al mask suspension devices to be totally incapable of meeting, and of teaching how to meet, the requirements and constraints imposed on the suspension devices in a system of the kind with which this invention is concerned.
The mask suspension systems of the referent copending applications have achieved noteworthy success in developmental tests in meeting the afore-described needs and requirements. This invention, however, represents an improvement over the systems of the said applications, as well as over the described prior art approach of Hofkenschied et al and all other known prior art systems.